The following tips for L4D2 have been compiled from various websites including youtube, steam forums and zombie command.

• Survivors move more slowly when walking in water, but infected are not affected.
• Adrenaline makes you heal, revive and pour gas faster, not get slowed down by infected attacks, and avoid the speed penalty of water.
• When reloading bash zombies away, it doesn't slow down the reload.
• Things like fences, busses, ledges, etc - you stand on the very edge hanging half your body off prevents infected from being able to grip that spot.
• If you have a common about to hit you, say you are running backwards and it is just gaining on you bash it away if you're going to get hit. People want to shoot all the time to try and kill things and it's not a natural instinct to bash (Not kill) first to save you taking some damage. You need to converse all the health you can when playing expert.
• Being polite to teammates and opponents improves teamwork and results in fewer ragequits. As a side effect, this may also cause a friend invite.
• Get a decent microphone and use the in-game comms, I've lost count of how many times a team mate has been smoked or jockied in the heat of the action and was unable to call for help.
• On realism mode, the witch will kill (not incapacitate) you in a single hit.
• Explosive ammo rounds can be used to repell active witches away from your team mates.
• Dont run off without your team, even if you think you are the best player in the world, if you get caught by a special there will be no rescue for you.  If you do it in a VS match your team will most likely vote kick you from the game.
• Incapacitated survivors cannot hurt teammates, so don't worry about hitting them whilst trying to keep the zombies off of them.
• As a survivor you can shoot the asphalt the tank throws to blow it apart in mid-air.
• If you're in front, crouch. That way people behind you can shoot over your head without shooting into your head.
• AI tanks go for the win now. Not the kill. He wants all four of you down. Not ONE of you dead.
• The chainsaw will only use fuel while your finger is held down on mouse1.
• Infected will chase after a bile bomb even if it kills them (for example, over the edge of a building or across a burning fire).
• Carnival games (to gain achievments) are best played under the influence of adrenaline.
• Protective ear muffs on construction worker zombies make them immune to pipe bombs.
• You can move healthpacks around maps by using a defibulator or special ammo and picking up and dropping the health and defibulator and walking backwards to where you want the healthpack to be (e.g. prior to finales or before starting survivor.)
• Tanks do a 2-4 seconds "taunt" like thing after downing someone, get a good 3-5 melee whacks per person in during that time (10 whacks kills it) [UNCONFIRMED]
• Keep moving forward .  Pace yourself though, too fast and you run into a shit-fest, too slow and one finds you!

• If you're playing versus and you're in a room with team mates for whatever reason (reviving friend, camping) and there's a cupboard/small room, you'll often find that a boomer/spitter spawns in there and takes you out, Sitting in the door way helps with this and makes the room safe, It allows you to cover the little room through view (the boomer won't be able to spawn) and help your friends because you can still shoot in the room.
• If a survivor is smoked, being pummelled by a charger or has a jockey riding him, you CANNOT inflict friendly fire damage. Feel free to unload on the afflicted survivor until he is free, even with a grenade launcher!
• If a smoker has someone and you free them make sure you kill the smoker or they'll come back and do it again, forcing a 25 second wait is the best thing you can do!
• As a spitter if you spit on a dropped gas tank (glowing orange) it will burst into flames, this works on scavenge and versus, the gas cans will then respawn where they were before the survivors picked them up.
• As a spitter, a Boomer is your best friend. Wait until the Boomer's horde stops the survivors in their tracks, and then spit on them. This hurts them severely without having the spit damage reduced such as when the survivor in question is pounced, pulled or grabbed.
• As a charger jumping mid-charge cancels your attack prematurely and avoids any collisions with walls etc, giving you a chance to swipe at the survivors you would of missed or take an escape route.
• In versus, there are several spots that are called the "no return zones" or "death spots". For example, in the carnival level, when the survivors have to drop through a hole in the floor. These spots are potentially deathly if you're using a smoker. You can try to hide and grab the last survivor when the rest have gone through the zone of no return, so you instakill him because his team cannot get back to help him.
• You can use Jockey's primary attack to perform small to medium jumps to cut distances through a map, kinda like the hunter's jumps, but less effective of course. You can also drop from buildings onto survivors without jumping.
• While playing as infected in versus, after the survivors leave the safe room you can choose to spawn there. (Something you could not done in L4D1)
• In VS if you're the tank you can go up to a closed safe room door and bash it open if they have closed it with a team mate outside to protect themselves. [UNCONFIRMED]
• If you're the tank don't attack a survivor if they're already pounced, smokered, jockied or chargered. You will only set them free and annoy the guy who had them incapped.

 The following image shows how effective each weapon is when taking on a tank in expert mode.